My driver's seat keeps sinking down

You don't need to put any weight on the seat to find a leak. Just air it all the way up and hold the button for 3.79 seconds after its aired all the way up. There will be enough pressure in the bags to detect a leak.

This isn't some kind of fancy computer controlled seat is it?

 

Dude’s got a leaky seat. Seat Air springs and switches go bad all the time. Seen some replaced under warranty. And yes a leaky seat can bring a parked air system down to nothing in no time. The whole air system is tied together. Put a shutoff valve on the supply line before the seat switch to keep the leak from bringing the rest of the system down in the future.

 

While standing on ground with cab door open and engine off Hit seat switch and fill seat up with air. Let it go all the why up. Spray bubble leak finder or soapy water all over the air spring, switch, line, and connectors. Look for suds that grow or bubbles. If none there then it may be were the supply line comes from a distribution manifold somewhere or at air tank fitting for it. Or the passage fitting thru the cab floor if it is routed that way.

 

When I rebuild air systems on the old stuff I work on the First thing I do is tap in a permanent shutoff valve into an air tank for shop air. For two reasons.
One I can have air to the whole system for checks and to run everything that uses air quietly for leak detection without having to fire the engine every time i need to recharge the system. 1000 times an hour.
Two instead of waiting 5 minutes for engine compressor to charge I can use shop air to charge the system and release brakes immediately to idle truck out of shop to let warm up outside and not fill shop with cold start smoke.

 

The same mixture you've always used when looking for air leaks on the truck will work fine.

 

Ok I want to play.
Proper soap to water ratio for making leak detection fluid per quart: 1 quart of distilled water, 3 tablespoons of dawn dish detergent or equivalent, add 2 teaspoons of liquid glycerin to make the bubbles last longer, which also makes them easier to detect. For mixing a pint, simply half amounts used.

 

I have found dish soap works best.

 

Did you ever stop to think that maybe the mechanic was fed up with your questions and opinions and decided to tell you whatever would get you the hell out his shop the quickest?
Mechanics, as a rule, are not known for their patience with people. Machinery, yes. They'll work on something as long as they need to to fix it but they draw the line at baby sitting recalcitrant rookies.

If it comes down to a choice a company will replace a driver before it replaces a mechanic. Good mechanics are hard to find.